You can and should use an HTML / plain text editor.
Please research making child themes before you start customizing a theme you will want to use on your site.
Unless you have WP installed locally you will need to edit your theme files and upload to your theme folder on your server.
Editing WP themes is not for the faint of heart LOL.. do some research in the codex.
Or make a child theme of say 2012, and play around with that to see how it all works together.

Thanks dragonsjaw ..when I extract the files, which of those file folders is usually the main template to load into the editor? Also it seems like this is not so “simple” by looking over these Codex pages, why do they talk about these wordpress themes to bloggers like it was falling off a log ..to put one up???